In the spread spectrum communication using a convolver, there is known e.g. a circuit indicated in FIG. 6, by which a correlation pulse is obtained from a correlation spike outputted by the convolver. In the figure, reference numerals 21 to 23 are a first, a second and a third threshold detector, respectively; 24 and 25 are monostable multi-vibrators; 26 is an up-down counter of N bits; 27 is a D/A converter; and 28 is a voltage divider.
In the circuit described above, voltages V.sub.A, V.sub.B and V.sub.C at outputs A, B and C of the voltage divider 28 are given to the detectors 21, 22 and 23, respectively, as threshold voltages, and the circuit works as indicated in the following table.
__________________________________________________________________________ OUTPUT OUTPUT STATE OF UP- STATE a b DOWN COUNTER 26 V.sub.A, V.sub.B, V.sub.C __________________________________________________________________________ 1 PRESENT PRESENT COUNT UP INCREASE 2 ABSENT PRESENT COUNT STOP MAINTAINED 3 ABSENT ABSENT COUNT DOWN DECREASE __________________________________________________________________________
As the result of the operation described above, a relationship between a correlation spike S.sub.1 and the threshold voltages V.sub.A, V.sub.B and V.sub.C, as indicated in FIG. 7(a), is valid and thus a threshold voltage V.sub.C suitable for restoring data so as to obtain a correlation pulse, as indicated in FIG. 7(b), is obtained.
However the prior art circuit construction has problems as described below.
1. Three threshold detectors are necessary.
2. .DELTA.V=V.sub.A -V.sub.B should be sufficiently great so that the output of the D/A converter 27 is not varied by noise, for the case where the S/N ratio of the received signal is low and noise is superposed on the correlation spike, which worsens the reliability.
3. In the case where FSK or CSK (Code Shift Keying) modulation is effected on the transmitter side and the receiver side is operated so as to follow the frequency (for FSK) or the PN code (for CSK), a control delay .tau. is produced, as indicated in FIG. 8, and a time zone where the frequency or the PN code is instantaneously in disaccordance that, is produced. In this time zone, since the output of the convolver disappears, the threshold levels V.sub.A to V.sub.C are decreased and therefore, when the output of the convolver appears again, the threshold level can be suitable no more.
In FIG. 8, PN represents a time-inverted code of the PN code.